Formation of a Copper Oxide Layer as a Key Step in the Metallic

J. Phys. Chem. C 2008, 112, 4275-4280

4275

Formation of a Copper Oxide Layer as a Key Step in the Metallic Copper Deposition Mechanism David Gimenez-Romero,† Jose´ Juan Garcı´a-Jaren˜ o,*,† Jero´ nimo Agrisuelas,† Claude Gabrielli,‡ Hubert Perrot,‡ and Francisco Vicente† Departament de Quı´mica Fı´sica, UniVersitat de Vale` ncia, C/ Dr Moliner, 50, 46100, Burjassot, Vale` ncia, Spain, and UPR 15 du CNRS, Laboratoire de Interfaces et Syste` mes Electrochimiques, UniVersite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France ReceiVed: NoVember 8, 2007; In Final Form: January 10, 2008

This paper demonstrates the formation of a Cu(OH)1.5Cl0.5 layer as a key step in the metallic copper electrodeposition mechanism in sulfate solutions. This layer is located surprisingly into the metal/solution interface and not on the reaction substrate. The difficult to study metal/solution interface makes indispensable the use of unconventional measurement techniques. In this way, a careful in situ study by means of acoustic impedance techniques associated with gravimetric techniques allowed the growth of this layer during the metallic copper electrodeposition process to be monitored, probably for the first time.

1. Introduction In recent decades, semiconductor materials have been researched intensively, principally because of their application in the microelectronic field.1-3 Since the first integrated circuit (IC) was invented in 1961 by Robert Noyce, the IC industry has been evolving at a remarkable place and it has been exceptionally successful in delivering operating speed and affordability to their customers. One primary emphasis of this evolution has been the increase of density of transistors. Thus, and due to the extremely high density of transistors, multilayers of interconnects are needed. The material of choice for interconnects has traditionally been the semiconductor Al/SiO24. Nonetheless, copper has replaced aluminum as the material of choice for interconnects in advanced integrated circuits.5 This transition is due to copper’s lower electrical resistance and higher electromigration resistance, both of which are required for deep submicrometer features currently in production.6 However, Cu integration poses a number of challenges: (a) it exhibits high diffusivity into Si, necessitating the use of barrier layers; (b) it is oxidized easily and the oxide layer does not form a self-passivation layer to limit further oxidization; and (c) moreover, adhesion of Cu to adjacent device layers is a significant reliability issue. Considering all of these challenges, the Damascene process has been adopted to allow copper metal to replace Al.5 Normally, the copper plating baths used for forming integrated circuit interconnects contain CuSO4 + H2SO4 with three or four component additive mixtures, which facilitate the superfilling of via holes and trench lines during damascene plating.7,8 In particular, chloride ions are used in most organic combinations of these additives.9 Metallic copper electrodeposition in sulfuric acid solution takes place through two consecutive charge-transfer steps involving the soluble intermediate Cu(I).10-15 This reaction * Author to whom correspondence should be addressed. E-mail: [email protected]. † Universitat de Vale ` ncia. ‡ Universite ´ Pierre et Marie Curie.

mechanism can be schematized through the following consecutive reactions

Cu2 + + (*) + e- f Cu + (*)

(1)

Cu + (*) + e- f Cu

(2)

where (*) represents the free sites on the electrode surface. The role of copper intermediates in the metallic copper electrodeposition reaction has long been acknowledged, but it is not yet fully understood. It has been shown that the addition of low quantities of chloride ions in a copper sulfate bath modifies the electrode kinetics by means of the stabilization of the cuprous ions, which form various types of complexes16 depending on the respective concentrations of chloride and cuprous ions. Some authors propose that the chloride ions are adsorbed on the copper electrode to form CuCl, which partially blocks the electrode surface and it is reduced to metallic copper.13 The amount of chloride ions added to plating baths is adjusted so that CuCl is not formed in the bulk bath.17 When the CuCl film coverage is low, the chloride ions enhance the copper deposition/dissolution rate given that they facilitate the access of Cu(II) to the electrode by bringing it to the metal.18 At this point, the use of electrogravimetric techniques (EQCM) in situ together with electrochemical techniques has revealed as a very powerful tool for the study of these processes of electrodeposition and dissolution of metals. The analysis of the mass/electrical charge ratio at each potential (F(∆m/∆Q) function) allows the identification of active species participating at the reaction.19-21 In addition, there is the possibility to follow in situ changes of motional resistance (acoustic impedance) for the EQCM electrodes. This resistance can be associated with changes of physical properties of the metal deposited on the surface electrode but also to changes of the viscoelastic properties of the interface in contact with the working electrode.22-24 The main goal of this work is to demonstrate the existence of a viscoelastic hydrated layer of copper intermediated species, which is formed prior to the metallic copper electrodeposition

10.1021/jp7107076 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/23/2008

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process. For that, pioneer techniques (i.e., in situ acoustic impedance) have been employed to monitor the physicochemical properties of the metal/solution interface. 2. Experimental Section The electrochemical measurements were carried out in a typical electrochemical three-electrode cell. The metallic copper was deposited on 25 mm2 diameter gold electrodes of a quartz crystal (Matel-Fordahl). A platinum plate was used as the counter electrode, and the Ag|AgCl|KClsat electrode was used as the reference electrode. The electrogravimetric measurements were realized through an AUTOLAB potentiostat-galvanostat (PGSTAT302). At the same time, the gravimetric and acoustic measurements were realized by means of an Electrochemical Quartz Crystal Microbalance (EQCM, RQCM, Maxtek Inc.). To monitor the structural variation of copper electrodepositions by means of acoustic fundaments, the RQCM utilizes an internal phase lock oscillator referred to as a voltage-controlled oscillator to drive the quartz crystal. The crystal current is monitored, and the frequency of the oscillator is adjusted until there is zero phase between the crystal voltage and current. At this point, the magnitude of the current is directly proportional to the crystal conductance. This current is monitored by the RQCM and displayed as motional resistance.25 Voltammetric experiments were carried out in the potential range from 0.80 to -0.60 V at scan rate ) 10 mV s-1. The working solution was CuSO4 (R.P. Normapur, p.a.) 10 mM, H2SO4 (Prolabo, p.a.) 0.1 M, pH ) 1.25. The solutions with chloride ions were composed of CuSO4 10 mM, H2SO4 0.1 M, and KCl (R.P. Normapur, p.a.) 50 mM, pH ) 1.25 and pH ) 4.02. Alternatively, these working solutions were deaerated by bubbling N2 (Air Liquide) for 5 min. At these experimental conditions, other nonideal contributions that could cause frequency changes, like roughness or viscoelastic effects, are not expected.27 3. Results and Discussion Electrogravimetry-dc. The metallic copper electrodissolution mechanism in aqueous sulfate solutions was established in a previous paper15 by the analysis of the mass/electrical charge ratio (F(∆m/∆Q) function). In chloride-containing solutions (Figure 1a), the metallic copper electrodissolution at these experimental conditions takes place via two single-electron transfers associated with the two voltammetric peaks of dissolution. Both peaks have the same electrical charge, and the sum is slightly smaller than the deposition charge (about 10%). The first single-electron transfer is accompanied by a mass increase due to the chloride ions’ adsorption, which stabilizes cuprous ions, the reaction intermediate. On the contrary, the second single-electron transfer is accompanied by a mass loss due to the desorption of these cuprous complexes. The copper electrodeposition mechanism in a sulfate solution with chloride ions will also take place through two singleelectron transfers.10-15 To study in depth this reaction mechanism, Figure 1a shows the evolution of the mass during the metallic copper electrodeposition process. The analysis can start by the fact that the faradaic current in zone 1 does not involve an increase of the mass deposited on the reaction substrate, ∆m ) 0 g, it is similar for chloride-free solutions (Figure 2a). The global mass/electrical charge ratio reaches values (28 g mol-1) slightly smaller than those expected -31.8 g mol-1 for a Cu(II) f Cu(0) reaction, F(∆m/∆Q) ) 63.5/-2 ) -31.8 g

Figure 1. Voltammetric scan of the metallic copper electrodeposition process. The working solution was CuSO4 10 mM, H2SO4 0.1 M, and KCl 50 mM, pH ) 1.25. (a) Current-mass response. (b) F(∆m/∆Q) function-motional resistance response.

mol-1 indicating so the possibility of a second reaction (the hydrogen evolution). The analysis at each potential of the F(∆m/∆Q) function can give more valuable information about the reaction mechanisms. Thus, Figure 1b characterizes by means of this function the metallic copper electrodeposition mechanism in a sulfate solution with chloride ions. Values of this function about 0 g mol-1 in zone 1 confirms that the species involved in the electron transfer occurring at these potentials are located in the same phase. This zone corresponds to the first single-electron transfer (Cu(II) f Cu(I)) because it takes place at the beginning of the cathodic scan. Because the cuprous ions adsorption is not detected (no mass increase), this first transfer takes place inevitably into the working solution given that it does not involve the phase change of their reactants. Consequently, the

Formation of a Copper Oxide Layer

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Figure 2. Voltammetric scan of the metallic copper electrodeposition process. The working solution was CuSO4 10 mM and H2SO4 0.1 M, pH ) 1.25. (a) Current-mass response. (b) F(∆m/∆Q) function-motional resistance response.

first reaction step of the copper electrodeposition mechanism in sulfate solutions with chloride ions corresponds to

Cusolution

2+

+ e- f Cu(I)solution

(3)

The cuprous ions formed from this reaction are stabilized into the solution due to the chloride complexes formation, as the bibliography indicates.13 In sulfate solutions with chloride ions, the F(∆m/∆Q) function also divides the potential range corresponding to the reaction substrate mass increase in two reaction zones: zones 2 and 3 of Figure 1b. In zone 2, the molecular weight of the deposited or adsorbed species is about -60 g mol-1. Because the F(∆m/ ∆Q) function has the same value that the atomic weight of copper atoms (63.5 g mol-1), it is possible to say that the metallic copper electrodeposition process at these potentials takes place only by means of a single-electron transfer. In consequence, the process that occurs at these potentials is the electrodeposition of metallic copper from the cuprous ions formed previously into the working solution (zone 1). The nonexistence of values of the F(∆m/∆Q) function below -30

g mol-1 from zone 2 confirms this fact. If the 1eCu(II)solution98Cu(I)substrate (F(∆m/∆Q) ) 63.5/-1 ) -63.5 g mol-1) reaction took place at this potential range, then the 1eCu(I)substrate98Cu(0)substrate (F(∆m/∆Q) ) 0/-1 ) 0 g mol-1) reaction should also be observed at more cathodic potentials, and therefore the values of the F(∆m/∆Q) function should be below -30 g mol-1. As a result, the voltammetric peak observed during the metallic copper electrodeposition process corresponds to the reduction of the excess cuprous ions formed at more anodic potentials. This result is confirmed easily by the Pourbaix-type diagrams.28 Last, the F(∆m/∆Q) function reaches a value about -30 g mol-1 in zone 3 of Figure 1b. This value corresponds simply to the global process of metallic copper electrodeposition, a two2eelectron transfer (Cu(II)solution98Cu(0)substrate, F(∆m/∆Q) ) 63.5/-2 ) -31.8 g mol-1). At these potentials, the rate of second single-electron transfer is higher than the rate of the first single-electron transfer, and as a result the metallic copper electrodeposition mechanism corresponds to a two-electron transfer.

4278 J. Phys. Chem. C, Vol. 112, No. 11, 2008 Figure 2 shows the same electrogravimetric study during the metallic copper electrodeposition process in a sulfate solution without chloride ions. Thus, this figure shows that the shape and values of the F(∆m/∆Q) function do not change sensibly with (Figure 1b) or without (Figure 2b) chloride ions in the working solution. Consequently, the species exchanged between the substrate and the solution during the copper electrodeposition process are not influenced to a large extent by these anions. As commented above, the chloride ions only stabilize the cuprous ions located in the working solution. This result is confirmed by Kelly et al.29 who showed that the chloride ions promote the metallic copper electrodeposition reactions but without the modification of the reaction mechanism. This stabilization process explains the different values of the F(∆m/∆Q) function in zones 1 and 2. Zone 1 in chloridecontaining solutions (Figure 1) shows a clear electrical current increase, which is not associated with a mass change, whereas this zone proves smaller in chloride-free solutions (zoom of Figure 2). In zone 1 of this last solution (Figure 2), the values of the F(∆m/∆Q) function, about -20 g mol-1, indicate the presence of both reactions. Alternatively, the F(∆m/∆Q) function in zone 2 of both solutions reaches values larger than those 2e-

expected for a Cu(II)solution98Cu(0)substrate (F(∆m/∆Q) ) 63.5/-2 ) -31.8 g mol-1) reaction (-40 g mol-1 in chloridefree solutions and -60 g mol-1 in chloride-containing solutions), indicating the disappearance of the cuprous ions on the surface of the working electrode. Accordingly, the two single-electron transfers of the electrodeposition mechanism are more separated over potential during the metallic copper electrodeposition process, which takes place in a solution with chloride ions due to the reaction intermediate stabilization. Acoustic Impedance. The acoustic behavior of a quartz resonator loaded with a thin layer can be described in terms of the Butterworth-Van Dyke (BVD) equivalent circuit model.22 The elements in this model are related to the physical properties of the crystal, the coating, and the solution. The equivalent circuit is constituted by a “motional” arm, incorporating three elements in series (a resistance Rm, an inductance Lm, and a capacitance Cm), which is in parallel with a “static” arm including a capacitance, Cs. It is well known that the electrical model can be converted to a mechanical model by means of the electromechanical coupling factor as an electromechanical analogy. Thus, the motional resistance (Rm) represents the energy dissipation due to the internal friction and damping from the crystal mounting. Lm is the initial mass/motional inertia of the system, Cm is the mechanical elasticity of the quartz, and Cs consists of the capacitance of the quartz between the electrodes and the parasitic capacitance of the crystal fixture. In a detailed model, the loaded resonator is a combination of a quartz crystal with two electrodes and a film around them. Accordingly, Granstaff et al.23 deduced a relation between the mechanical properties of the different components of this piezoelectric system and the impedance of the “motional” arm of the BVD equivalent circuit model. From this relation, a relation between these mechanical properties and the motional resistance of the BVD equivalent circuit model was deduced in a previous paper,24 allowing the physical characteristics of the materials in contact with the piezoelectric material to be monitored. This relation corresponds to

Rm )

π2ηquartzdquartz 8K2c66ξ22A

+

Gimenez-Romero et al.

(

2 ' d2quartz ω2F3/2 filmdfilm Gfilm

4e226A (G''film)2 - (G'film)2 d2quartz 4e226A

x

)

+

ωFsolutionηsolution (4) 2

where d is the layer thickness, F the mass density, ω is the angular frequency, η is the viscosity, and G is the complex shear modulus. K2 is the electromechanical coupling factor for lossless quartz (7.74 × 10-3),23 ηquartz is the effective viscosity of quart (3.5 × 10-4 kg m-1 s-1),23 c66 is the piezoelectrically stiffened elastic constant for lossless quartz (2.947 × 1010 N m-2),23 ξ22 is the quartz permittivity (3.982 × 10-11 A2 s4 kg-1 m-3),23 e26 is the piezoelectric stress constant for quartz (95.3 × 10-3 A s m-2),23 and A is the active electrode area. The complex shear modulus of the film is equal to G′film + jG′′film(j ) x-1). It is important to emphasize that G′film , G′′film for a viscoelastic layer and G′film . G′′film for a rigid film. Thus, an increase of the density or thickness of the film implies the increase of the motional resistance in viscoelastic films as well as the decrease of the motional resistance in rigid films. The motional resistance is the addition of four different terms: the resistances related to the properties of the quartz crystal, the working solution, the electrolyte, and the film (the addition of the different resistances associated with the different layers that compose the reaction system). Thus, eq 4 is transformed in

Rm )

π2ηquartzdquartz 8K2c66ξ22A

-

d2quartz

(

(

)

2 d2quartz ω2F3/2 Cu dCu + 4e2 A G′Cu 26 2 ω2F3/2 filmdfilmG′film

4e226A (G′′film)2 - (G′film)2 d2quartz 4e226A

x

)

+

ωFsolutionηsolution (5) 2

where the “Cu” indication designates the physical properties of the metallic copper and the “film” indication designates the physical properties of the layer located around the metallic copper. It is important to emphasize that the viscoelastic materials have higher values of the acoustic impedance than the rigid materials. Consequently, if there is a viscoelastic material near the microbalance substrate, then the measured motional resistance corresponds for the most part to the contribution of the viscoelastic material. Thus, this resistance can be simplified as follows:

Rm )

d2quartz

(

2 ω2F3/2 filmdfilmG′film

4e226A (G′′film)2 - (G′film)2

)

(6)

In view of that, Figure 1b shows the change of the motional resistance measured during the growth of the metallic copper film in a sulfate solution with chloride ions. At these experimental conditions, the motional resistance increases during the whole electrodeposition process. However, and given that the metallic copper is a rigid material, the motional resistance should decrease if only the metallic copper deposition growth took place. Thus, the motional resistance increase indicates the formation of a viscoelastic layer during the metallic copper

Formation of a Copper Oxide Layer

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Figure 4. Blank voltammogram of a gold EQCM electrode in a H2SO4 0.1 M and KCl 50 mM, pH ) 1.25 solution. Figure 3. Current-motional resistance response of the metallic copper electrodeposition process. The working solution CuSO4 10 mM, H2SO4 0.1 M, and KCl 50 mM, pH ) 4.02.

electrodeposition process. This detected viscoelastic layer must be constituted by cupric ions because it begins its growth from zone 2. The metallic copper cannot generate this viscoelastic layer given that it is a rigid material, and at the same time the cuprous ions cannot generate it either because their concentration decreases at these potentials where the viscoelastic layer growth is observed, Figure 1b. Accordingly, the acoustic techniques by means of the motional resistance allow the growth of a viscoelastic layer during the metallic copper electrodeposition process in a sulfate solution with chloride ions to be monitored in situ. Figure 3 shows the same experiment as that Figure 1b, but at a higher pH (4.02). It is important to emphasize that changes of motional resistance are similar in zone 2 of both Figures, but Rm changes abruptly in zone 3 of Figure 3 (pH ) 4.02) while it keeps constant in zone 3 of Figure 1b (pH ) 1.25). Therefore, the viscoelastic layer formed in zone 2 may be attributed to cupric ions and chloride ions because it is observed mainly in solutions containing Cl- anions and it does not change with the pH of the solution. The formation of this layer at these potentials will take place due to the cuprous ions’ reduction and, in consequence, to the increase of the chloride and cupric ion concentration near the reaction substrate. In contrast, the layer formed in zone 3 is constituted by the cupric, hydroxyl, and chloride ions given that the motional resistance change in this potential range is influenced by all of these species. As a result, the layer at these potentials could be constituted by Cu(OH)1.5Cl0.5, the species postulated from the Pourbaix-type diagrams.28 This species is in agreement with the formation of copper chloride species associated with copper oxide in presence of chloride ions.30,31 Thus, the formation of this layer will be due to the local basification of the interface zone by the hydrogen evolution that takes place at these potentials as is possible to observe in Figure 4 and also due to the fact that the global mass/electrical charge ratio reaches values slightly smaller than those theoretically expected for a pure Cu(II) f Cu(0) reaction. It is important to highlight that this layer is detected by changes of the motional resistance while the F(∆m/∆Q) function indicates only the deposition of metallic copper atoms on the

reaction substrate (a rigid material). Therefore, it is possible to say that the formed layer is located in the metal/solution interface and not on the reaction substrate. Thus, the motional resistance only detects the viscoelasticity change of the interfacial layer located around this substrate; this phenomenon is known in the bibliography as depletion layer effects.32,33 Finally and in spite of the fact that the experimental conditions in the Damascene process are slightly different to our conditions, these results are of interest to this important industrial process. 4. Conclusions A Cu(OH)1.5Cl0.5 layer was found to be determinant in the metallic copper electrodeposition mechanism in sulfate solutions. This new layer is surprisingly located in the metal/solution interface zone and not on the reaction substrate. Accordingly and probably for the first time, the growth of this hydrated layer has been monitored by means of the acoustic techniques and thus can make its characterization easier. In next to no time, the characterization of this viscoelastic layer will allow the physical characteristics of the metallic copper layers deposited by electrochemical techniques to be controlled, with the technological importance that entails. Acknowledgment. This work was supported by FEDERCICyT project CTQ2007-64005/BQU. D.G.-R. acknowledges his position to the Generalitat Valenciana. References and Notes (1) Munoz, E. C.; Schrebler, R. S.; Cury, P. K.; Suarez, C. A.; Cordova, R. A.; Gomez, C. H.; Marotti, R. E.; Dalchiele, E. A. J. Phys. Chem. B 2006, 110, 21109. (2) Garcia-Jaren˜o, J. J.; Gimenez-Romero, D.; Keddam, M.; Vicente, F. J. Phys. Chem. B 2005, 109, 4584. (3) Gimenez-Romero, D.; Garcia-Jaren˜o, J. J.; Vicente, F. J. Electroanal. Chem. 2004, 572, 235. (4) Mitra, A.; Cao, T.; Wang, H.; Wang, Z.; Huang, L.; Li, S.; Li, Z.; Yan, Y. Ind. Eng. Chem. Res. 2004, 43, 2946. (5) Edelstein D.; Heidenreich J.; Goldblatt R.; Cote W.; Uzoh C.; Lustig N.; Roper, P.; McDevitt T.; Motsiff W.; Simon A.; Dukovic J.; Wachnik R.; Rathore H.; Schulz R.; Su L.; Luce S.; Slattery J. Technical Digest, IEEE International Electron DeVices Meeting, 1997; p 773. (6) Zong, Y.; Shan, X.; Watkins, J. J. Langmuir 2004, 20, 9210. (7) West, A. C.; Mayer, S.; Reid, J. Electrochem. Solid State Lett. 2001, 4, C50. (8) Vereecken, P. M.; Binstead, R. A.; Deligianni, H.; Andricacos, P. C. IBM J. Res. DeV. 2005, 49, 3.

4280 J. Phys. Chem. C, Vol. 112, No. 11, 2008 (9) Bonou, L.; Eyraud, M.; Denoyel, R.; Massiani, Y. Electrochim. Acta 2002, 47, 4139. (10) Oskam, G.; Long, J.; Nataranj, A.; Searson, P. C. J. Phys. D 1998, 31, 1927. (11) Mattson, E.; Bockris, O. M. Trans. Faraday Soc. 1959, 55, 1586. (12) Bokris, O. M.; Enyo, M. Trans. Faraday Soc. 1962, 58, 1187. (13) Gabrielli, C.; Moc¸ ote´guy, P.; Perrot, H.; Wiart, R. J. Electroanal. Chem. 2004, 572, 367. (14) Nun˜ez-Flores, M. A.; Ruano, A.; Vicente, F. An. Quim. 1988, 84, 289. (15) Gimenez-Romero, D.; Gabrielli, C.; Garcı´a-Jaren˜o, J. J.; Perrot, H.; Vicente, F. J. Electrochem. Soc. 2006, 153, J32. (16) Soares, D. M.; Wasle, S.; Weil, K. G.; Doblhofer, K. J. Electroanal. Chem. 2002, 532, 353. (17) Goldbach, S.; Messing, W.; Daenen, T.; Lapicque, F. Electrochim. Acta 1998, 44, 323. (18) Gabrielli, C.; Moc¸ ote´guy, P.; Perrot, H.; Nieto-Sanz, D.; Zdunek, A. Electrochim. Acta 2006, 51, 1462. (19) Gimenez-Romero, D.; Garcia-Jaren˜o, J. J.; Vicente, F. J. Electroanal. Chem. 2003, 5, 722. (20) Gregori, J.; Garcia-Jaren˜o, J. J.; Gimenez-Romero, D.; Vicente, F. J. Electrochem. Soc. 2006, 153, B206.

Gimenez-Romero et al. (21) Gimenez-Romero, D.; Garcia-Jaren˜o, J. J.; Vicente F. Electrochem. Commun. 2004, 6, 903. (22) Parzen B. Design of Crystal and Other Harmonic Oscillators; Wiley: New York, 1983. (23) Granstaff, V. E.; Martin, S. J. J. Appl. Phys. 1994, 75, 1319. (24) Gimenez-Romero, D.; Agrisuelas, J.; Garcia-Jareno, J. J.; Gregori, J.; Gabrielli, C.; Perrot, H.; Vicente, F. J. Am. Chem. Soc. 2007, 129, 7121. (25) Operation and SerVice Manual of RQCM, 2nd ed.; Maxtek Inc. (26) Gileadi E.; Tsionsky V. J. Electrochem. Soc. 2000, 147, 567. (27) Garcı´a-Jaren˜o, J. J.; Gabrielli, C.; Perrot, H. Electrochem. Commun. 2000, 2, 195. (28) Nila, C.; Gonzalez, I. Hydrometallurgy 1996, 42, 63. (29) Kelly, J.; West, A. J. Electrochem. Soc. 1998, 145, 3472. (30) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed.; NACE: Houston, 1974; p 390. (31) Kunze, J.; Maurice, V.; Klein, L. H.; Strehblow, H. H.; Marcus, P. Corros. Sci. 2004, 46, 5. (32) Lee, W. W.; White, H. S.; Ward, M. D. Anal. Chem. 1993, 65, 3232. (33) Xie, Q.; Wang, J.; Zhou, A.; Zhang, Y.; Liu, H.; Xu, Z.; Yuan, Y.; Deng, M.; Yao, S. Anal. Chem. 1999, 71, 4649.